Alkaline cells are well known in the art and generally employ a zinc anode, manganese dioxide as the cathode with an aqueous solution of potassium hydroxide for the electrolyte. These cells are readily available commercially for industrial and home applications. Recently a new type of alkaline cell was disclosed by Cegasa International, a Spanish company. This cell, referred to as an air-assisted cell, employs zinc as the anode and manganese dioxide as the cathode with an aqueous solution of potassium hydroxide as the electrolyte. This cell is designed so that the positive electrode containing the manganese dioxide (MnO.sub.2) is supported about its periphery and along its full length in the cell by a perforated ribbed air distribution grid. The bottom or negative end of the cell has an insulating support which allows air to enter the cell and pass up along the outside of the supported positive electrode. When the cell is initially Put into a circuit, the electrochemical reaction depends primarily upon the presence of the manganese dioxide cathode. As the reaction progresses, and the manganese dioxide cathode is electrochemically reduced, air within the cell reoxidizes and recharges the manganese dioxide. Thus an air-assisted cell is designed to use oxygen in the air to "recharge" manganese dioxide in the cathode. This "recharging" of the manganese dioxide means that the fixed quantity of manganese dioxide in the cathode can be discharged and then recharged numerous times. In contrast, the cathode's ampere hour output in a standard alkaline battery is limited by the quantity of manganese dioxide incorporated in the cell when the cell is manufactured. Therefore, based upon the cathode's ampere hour input, the maximum service obtainable from an air-assisted alkaline battery is greater than the maximum service which can be obtained from a comparably sized standard alkaline battery. The need to get oxygen to the manganese dioxide in an air-assisted alkaline cell means that a portion of the battery, such as the seal, must be designed to allow oxygen to flow through and directly contact the cathode. Contrary to this, the seal in regular alkaline cells is designed to be air tight.
Standard alkaline batteries and air-assisted alkaline batteries have traditionally been made with mercury in the anode. Mercury helped to prevent gassing by raising the hydrogen overvoltage. Due to concerns about the environment, battery manufacturers are designing alkaline batteries with little or no mercury. In the hermetically sealed alkaline batteries, gassing can be contained because the cell is sealed. However, the "open" design of air-assisted alkaline cells prevents containment of gas within the cell. As the percentage of mercury in the cell has been reduced, Problems with leakage and decreased shelf life have become apparent. The reduction or elimination of mercury has led to service and shelf problems because gassing in the anode compartment has forced electrolyte to flow from the anode compartment into the cathode compartment. As electrolyte moves from the anode into the cathode, ionic conductivity in the anode decreases and the cell cannot discharge efficiently. At the same time, pores in the cathode become flooded thereby preventing oxygen from contacting and recharging the manganese dioxide. A severe drop in the cell's service is the net result. If the anode compartment continues to gas and drive electrolyte from the anode and into the cathode, then the electrolyte may eventually be driven through the seal and out of the air access opening in the cover.
It is an object of the present invention to provide an air-assisted cell with improved leakage characteristics while maintaining good service and shelf performance.
It is another object of the invention to provide means for releasing hydrogen from an air-assisted cell prior to unacceptable pressure buildup within the cell and means to retain the anode's electrolyte within the confines of a separator encasing the anode.
It is another object of the present invention to provide an air-assisted cell, having reduced or no mercury in the anode, with good service and shelf performance and which is easy to make and cost effective to produce.
The above and further objects will become apparent upon consideration of the following description and drawings thereof.